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AN703  Vishay Siliconix AN703 Designing DC/DC Converters with the Si9110 Switchmode Controller In distributed power systems and battery-powered equipment, connected through a diode to the power bus to provide the advantages of MOS over bipolar technology for pulse- emergency back-up. The distributed power approach is width modulation (PWM) controllers are significant. First, by employed in telecom systems, large minicomputers, and in using a BiC/DMOS power IC process, a high-voltage DMOS other applications where reliability is a primary concern. transistor can be integrated with a CMOS PWM controller to serve as a pre-regulator stage. This reduces the number ofTDoM ilOluSs trsawtiet cshommoed eo f ctohnet rpoellrefro rImC,a nac e1 5c-aWpa fboilritwieasr do f ctohnisv eBritCe/r external components by permitting the power controller IC to interface directly to the power bus. doeutsipgunt si sfr oprme sa e9n-t etod . 3T6h-Ve  icnopnuvt erratenrg ep.r oTvhiidse ps e+r5m-itVs  athned  p ± o12w-eVr The second advantage of MOS is speed. Bipolar PWM supply to operate from 12-V or 24-V batteries, or from a 28-V controllers can be made fast, but only with a significant aircraft power source. Before describing the forward converter increase in supply current. Logic gate delays of 5 ns are example, it is instructive to review the operation of each of the readily achievable using 5-µm CMOS, comparator Si9110 switchmode controller™s functional blocks. propagation delays are in the 50- to 100-ns range, and the supply current is maintained below 1 mA. FUNCTIONAL DESCRIPTION How does speed translate into power supply performance? The answer is first in reliability and second in power density. If Pre-Regulator the delay time is long between the sensing of an overcurrent A BiC/DMOS power integrated circuit process is used t condition in the power switch and the turn-off of the switch,integrate a high-voltage (120-V rated) lateral DMOS transis toor then the peak and RMS current values reach excessive levels with the CMOS PWM controller. By using an ion implant to and the switch fails. A well-designed power supply should shift the gate threshold to a negative value, as sh in tolerate a continuous short circuit on any output. ToFigure1, the transistor is made t operate as a depolewtnio n-accomplish this with a slow controller IC, extra protection o circuitry or an oversized switching transistor and heatsink areamboodvee  dV I e N v itcoe .t uTrhni ts heel idmeivniactee so nt,h ea nnde eand  afomr pali fipeur lla-unpd  vvoollttaaggee required. But that costs money. referenc n b used to implement a linear regulator, as e ca e Power supply density (often expressed as output power in shown in Figure 2. The CMOS circuitry is thus protected from watts divided by volume in cubic inches) has steadily been transients which appear on the input power bus.  increasing over the past 5 to 10 years. By increasing the switching frequency, the size of magnetics and filter capacitors has been reduced, allowing smaller and less expensive power supplies to be built. To increase the switching frequency to the 100- to 500-kHz range and still achieve high reliability requires that the current limit delay time be kept under approximately 100 ns. The first BiC/DMOS switchmode controller IC to meet these requirements is the Si9110. Its 500-kHz rating for maximum switching frequency is fully usable, thanks to the high-speed current limit comparator and the efficient output driver stage, which essentially eliminates the shoot-through current found in bipolar totem-pole circuits. The DMOS transistor in the input pre-regulator has a breakdown voltage rating of 120 V, which provides ample headroom for operation from typical bus voltages in distributed power systems (where 12, 24, 48, and 60 V are frequently encountered). The appeal of such distributed power processing systems is in their flexibility and reliability. By bussing power at a higher voltage, smaller conductors can be used, as well as fewer connector pins to get the power to where it is needed-on the circuit card. An on-card power supply can then provide the voltages needed in that part of the system. The power bus voltage is usually chosen to be low enough to eliminate the need for safety agency approvals, and a battery can be
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FIGURE 1. Depletion-Mode MOSFET Characteristics
FIGURE 2. Pre-regulator/Start-up Circuit
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